It is not uncommon for a vertical well to encounter a plurality of hydrocarbon bearing formations with varying degrees of potential productivity. Due to differences in reservoir pressure, fluid content, and petrophysical properties, downhole commingling of production from multiple zones if often not only detrimental to the ultimate recovery of the well, but prohibited by government regulatory agencies.
A number of different completion methods have been used to independently produce multiple zones encountered in a single well. In the simplest of these completion techniques, the lowermost productive zone is perforated and produced until the hydrocarbon production rate becomes economically marginal. Then, the zone is abandoned and the well is recompleted to the next shallower zone. Upon depletion of this zone, the well is again recompleted to the next shallower zone. Upon depletion of this zone, the well is again recompleted and produced until all potential zones have been produced. Upon depletion of the shallowest productive zone, the well is plugged and abandoned. A graph showing hydrocarbon production rate versus time for such a well would typically exhibit a "roller coaster" profile with relatively high production rates occurring immediately after each new zone completion.
In an effort to prolong a well's flush production period and smooth out this "roller coaster" production profile, more complex completion methods are employed. One such technique involves using multiple strings of production tubing with specially spaced multiple completion packers for isolating each completed zone. An important drawback to this type completion design is the size of independent production strings make it difficult to artificially lift the produced fluids from each zone should the well cease to flow naturally.
Multi-zone techniques facilitating the independent completion of one or more horizontal drainholes extending from a vertical well together with one or more "conventional" vertical well completions have become important to the oil industry in recent years. Such wells are commonly referred to as multi-lateral wells. Horizontal drainhole completions typically exhibit substantially better productivity than vertical well completions, but due to the increased well cost coupled with the requirement of excellent subsurface geologic definition, are not appropriate in all cases. Horizontally drilled wells, or wells which have nearly horizontal sections, are now used routinely to exploit productive formations in a number of development situations. Horizontal drainholes are often used to efficiently exploit vertically fractured formations, thin reservoirs having matrix porosity, formations prone to coning water, steam, or gas due to "radial flow" characteristics inherent in vertical well completions, and formations undergoing enhanced oil recovery operations. Drilling horizontal wells also has application in offshore development where fewer and smaller platforms are required due to the increased productivity of horizontal drainholes compared to vertical completions and the possibility of drilling multiple drainholes from one vertical well platform slot. Drilling multiple drainholes from a new or existing cased vertical well with completions in the same formation or in different formations enables both the productivity and return-on-investment in equipment infrastructure of the vertical well to be maximized.
The majority of multi-lateral wells drilled today are rather simply completed in the sense that the horizontal drainholes commingle well fluids in a vertical part of the well. The commingled fluids either flow or are artificially lifted from the vertical part of the well by equipment located substantially above the uppermost drainhole and productive formation(s). With this wellbore configuration, zone isolation, flow control, pump efficiency, and bottomhole pressure optimization is compromised. In some cases, downhole pumps are actually placed in the horizontal sections of the wells which partially remedies some of these problems, but typically leads to increased mechanical problems. When zone and/or drainhole isolation and flow control means are not incorporated in the well design, the entire well's production may be jeopardized if a production problem such as early water breakthrough occurs in one of the vertical well or drainhole completions.
In recent years, several more sophisticated multi-lateral drilling and completion techniques have been developed in an attempt to solve a host of difficult problems. It is well documented that the ideal multi-lateral system would overcome the shortcomings of the prior art and provide the following benefits: (1) infrastructure related to a cased vertical well should be used to efficiently deplete all economically productive zones with a series of vertical well completions and horizontal drainhole completions, (2) existing vertical wellbores with large diameter production casing should be re-enterable as the parent well for subsequent multi-lateral drilling and completion, (3) relatively simple design execution should be both cost effective and mechanically reliable, (4) should be applicable to short radius (ie: 60' turning radius) as well as medium radius (ie: 300' turning radius) drainholes used in high temperature enhanced oil recovery operations, (5) should not involve milling of "hard-to-drill" steel tubular goods to exit the cased vertical well for drainhole extension, (6) curve sections should be isolated from the horizontal target sections in drainholes to avoid hole collapse problems and/or premature gas or steam breakthrough, (7) light weight and flexible zone isolation and/or sand control liners should be installed in the horizontal target intervals of drainholes as well conditions dictate, (8) the size of the liner within each drainhole should be approximately equal to the final size of the production casing or liner string within the parent vertical wellbore, (9) the junction between the cased vertical well and each cased lateral well should be effectively sealed, (10) each vertical and/or horizontal well completion should be isolated within the vertical wellbore, (11) openable flow control devices should be employed to enable each completion to be selectively tested, stimulated, produced, or shut-in, (12) each drainhole should be accessible for re-entry to facilitate additional completion work, drilling deeper, drainhole interval testing with zone isolation, sand control, cleanout, stimulation, and/or other remedial work, and (13) the inside diameter of the final production casing or liner string in the vertical wellbore should be large enough to enable a downhole pump may be placed in a sump located below all productive horizons to optimize pressure drawdown during production operations and increase artificial lift efficiency. To date, a prior art multi-lateral drilling and completion system has not been developed that delivers all of the benefits described above.
U.S. patents of general interest in the field of horizontal well drilling and completion include: U.S. Pat. Nos. 2,397,070; 2,452,920; 2,858,107; 3,330,349; 3,887,021; 3,908,759; 4,396,075; 4,402,551; 4,415,205; 4,444,276; 4,573,541; 4,714;117; 4,742,871; 4,800,966; 4,807,704; 4,869,323; 4,880,059; 4,915,172; 4,928,763; 4,949,788; 5,040,601; 5,113,938; 5,289,876; 5,301,760; 5,311,936; 5,318,121; 5,318,122; 5,322,127; 5,325,924; 5,330,007; 5,337,808; 5,353,876; 5,375,661; 5,388,648; 5,398,754; 5,411,082; 5,423,387; and 5,427,177.
Of particular interest to this application is U.S. Pat. No. 5,301,760. According to this patent, a vertical well is drilled through one or more horizontal well target formations. The borehole may be enlarged adjacent to each proposed "kick-off point" prior to running and cementing production casing. An orientable retrievable whipstock/packer assembly (WPA) is used to initiate milling a window through a "more drillable" joint in the vertical well casing string in the direction of the proposed horizontal well target. A horizontal drainhole is then drilled as an extension of the vertical well. The drainhole is then completed with a cemented liner extending at least through the curve portion of the drainhole and into the vertical well. The protruding portion of the liner and cement in the vertical well is then removed using a full gauge (fitted to the vertical well casing inside diameter) burning shoe/fishing tool assembly. The resulting drainhole entrance point has an elliptical configuration with a sharp apex at the top of the liner and at the bottom of the liner at the junction of the lateral well with the vertical well due to the high angle (almost vertical) of the drainhole liner as it meets the vertical well. Furthermore, the "smooth" junction of the vertical well casing and the drainhole liner is effectively sealed by a highly resilient, impermeable cement sheath completely filling the annulus of the drainhole and the liner at the junction. Subsequent to "coring" through and removing the protruding portion of drainhole liner and cement in the vertical well, the WPA is removed from the well, thus re-establishing the full gauge integrity of the vertical well to enable large diameter downhole tools to be lowered below the drainhole entrance point. Additional drainholes may be drilled as extensions from the vertical parent well in a similar fashion.
Another U.S. patent of particular interest to this application is U.S. Pat. No. 5,289,876. According to this patent, one or more drainholes are drilled and completed using a method such as that described in U.S. Pat. No. 5,301,760 in junction with a novel method for preventing drainhole collapse, isolating lateral intervals drilled out-of-the-target zone, and providing sand control for laterals drilled through unconsolidated sands or incompetent formations. A light weight, flexible, "drillable" liner assembly is used to facilitate gravel packing with high temperature resistant curable resin coated sand. Subsequent to pumping the gravel pack, the "drillable" drainhole liner together with a veneer of cured resin coated sand adjacent to the target horizon is removed using a coil tubing conveyed mud motor and pilot mill. A liner with an inside diameter slightly larger than the outside diameter of the pilot mill is placed adjacent to the lateral intervals drilled out-of-the-target zone to isolate these intervals. The method disclosed in this patent is applicable to short and medium radius horizontal wells used in high temperature enhanced oil recovery operations.
Multi-lateral wells drilled and completed using the method disclosed in U.S. Pat. No. 5,289,876 in conjunction with the techniques described in U.S. Pat. No. 5,301,760 provide nine of the thirteen beneficial attributes previously described for the ideal multi-lateral system, namely: (1), (2), (3), (4), (5), (6), (7), (9), and (13). A need presently exists for a reliable and cost effective drilling and completion system for multi-lateral wells that addresses all thirteen previously described benefits. Accordingly, it is an object of the present invention to enhance the utility of the methods disclosed in U.S. Pat. Nos. 5,289,876 and 5,301,760 by allowing: (a) each vertical and/or horizontal well completion to be isolated within the vertical wellbore, (b) openable flow control devices to be employed to enable each completion to be selectively tested, stimulated, produced, or shut-in, (c) each drainhole to be selectively accessible for re-entry to facilitate additional completion work, drilling deeper, drainhole interval testing with zone isolation, sand control, cleanout, stimulation, and other remedial work either before or after completion isolation and flow control means are installed, and (d) the size of the liner within each drainhole to be approximately equal to the final size of the production casing or liner string within the parent vertical wellbore.